One of our favourite ESOTU Community Ambassadors and YouTuber/Streamer has posted a current video on the essentials of crafting writs. If you are wondering about writs and want a good primer, her video is worth the view. Duration: ~11 minutes.
Essentially, these crates are gambling with your real money to obtain crown store items no longer available or crown crate exclusives. Before gambling you might want to check your odds. An official crown crate thread has been created on the official forums.